The present invention relates to the pyrolysis of 2-sulfochloride benzoates, and, more especially, relates to the synthesis of o-sulfobenzoic anhydride by pyrolysis of the 2-sulfochloride benzoates in a continuous process and without the formation of undesirable by-products.
In the usual procedure for the synthesis of o-sulfobenzoic anhydride, acid amonium o-sulfobenzoate (conveniently prepared by the acid hydrolysis of saccharin) is reacted with excess thionyl chloride, typically in a toxic organic solvent such as benzene or the aromatic halides, e.g., chlorobenzene. See H. T. Clark and E. E. Dreger, Org. Syn. Coll. Vol. I, Second Edition, page 495 (1941). This method, which may be represented as follows: ##STR1## exhibits the disadvantage that the by-product ammonium chloride which precipitates during the course of the reaction must be filtered off from the reaction mixture while it is still hot in order to prevent premature crystallization of the anhydride. And this has proved to be particularly difficult by reason of the noxious fumes evolved. It will be appreciated, therefore, that it is also difficult to attain high yields or large amounts of anhydride product by this method, notably because any crystallized anhydride will be removed along with the precipitated ammonium chloride. Moreover, this process is batch in nature and consequently usually requires a minimum of about one full day for completion.
The more apparently obvious route, namely, that of dehydrating o-sulfobenzoic acid, is not as facile as at first blush the skilled artisan might expect. One method which employs this procedure involves azeotropic removal of water via distillation in relatively elaborate equipment with benzene in the presence of another organic liquid such as isobutyric acid. This alternate synthesis may be represented as follows: ##STR2## See Belgian Patent No. 629,100.
But, in any event, essentially pure o-sulfobenzoic acid free of cations is exceedingly tedious and difficult to prepare economically. One way to prepare o-sulfobenzoic acid is to start with saccharin; and this method obviously cannot be used for the ultimate synthesis of saccharin.
Ortho-sulfobenzoic anhydride, C.sub.6 H.sub.4 COOSO.sub.2, is a solid having a melting point of 129.5.degree. C., a boiling point of from 184.degree. to 186.degree. C. (18 mm.), and is soluble in hot water, ether, and benzene. It is used as a polymerization inhibitor. But o-sulfobenzoic anhydride is also a useful chemical intermediate and can, for example, be utilized in the manufacture of sulfophthalein indicators and dyes, and saccharin. Saccharin, of course, is variously known as o-benzosulfimide; gluside; benzoylsulfonic imide; and is the anhydride of o-sulfimide benzoic acid having the formula: ##STR3## Saccharin is employed in the manufacture of syrups, medicine (substitute for sugar), soft drinks, foods, and the like, and is a nonnutritive sweetener which can readily be converted to sodium or soluble saccharin, and is a white, crystalline powder. It has an exceedingly sweet taste (500 times that of cane sugar), a melting point of about 226.degree. to 230.degree. C., and is soluble in amyl acetate, ethyl acetate, benzene and alcohol; slightly soluble in water, chloroform and ether.
Saccharin has been made from toluene by the following series of reactions: ##STR4## The imide is converted to the sodium salt to increase the solubility in water. Saccharin can also be prepared by converting a mixture of toluene sulfonic acids into the sodium salt, then by distillation with phosphorus trichloride and chlorine to obtain the ortho-toluene sulfonyl chloride, which by means of ammonia is converted into ortho-toluenesulfamide. This is oxidized with permanganate, treated with acid and saccharin crystallized out. It is reported that the slight bitter taste associated with the saccharin prepared by either of the above methods is caused by the presence of o-toluamide. Moreover, the disposal of the p-toluenesulfonyl chloride obtained by-product in the above processes has also been a problem.
Both of these objections have been attempted to be overcome by two or more recent processes, the first [A] commencing with thianaphthene (prepared from styrene and sulfur) and the other [B] with anthranilic acid, as follows [see Noller, Chemistry of Organic Compounds, 2nd Edition, pp. 556-557 (1957)]: ##STR5## Nevertheless, it too is reported that even when utilizing these alternate routes there still is a slight bitter taste associated with the saccharin, here probably caused by the presence of certain trace contaminants.